Preparation of epoxides from alkanes using lanthanide-promoted silver catalysts

ABSTRACT

A process is provided for use in the conversion of alkanes into alkylene oxides, having particular utility in the conversion of propane to form propylene oxide, using a lanthanide-promoted, supported, silver catalyst prepared via precipitation. A preferred embodiment uses silver nitrate and lanthanum nitrate to form the catalyst on a BaCO3 support.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 09/792,390, filed onFeb. 22, 2001 now U.S. Pat. No. 6,509,485B2.

TECHNICAL FIELD

This invention relates generally to novel catalysts for use in thedirect epoxidation of alkanes to form alkylene oxides, having particularutility in the conversion of propane to propylene oxide. The catalyst isprepared via precipitation and is a lanthanide-promoted, supportedsilver catalyst. The invention also relates to methods for catalyzingoxidative chlorination/halodehydrogenation and epoxidation reactionsusing the novel catalyst, and to methods for manufacturing the novelcatalysts. The invention finds utility in the fields of catalysis.

BACKGROUND

Conversion of alkanes to oxygenates is typically considered to proceedvia oxidative dehydrogenation of an alkane to an alkene followed byepoxidation of the alkene to provide an oxide in a separate process. Adirect method of synthesizing alkylene oxides from an alkane such aspropane has not been practical heretofore as activation of the propanerequires high temperatures, which decompose partial oxidation products,particularly propylene oxide, or promote total oxidation.

Numerous studies have been conducted investigating the oxidativedehydrogenation of alkanes to produce alkenes. See, for example, Chaaret al. (1988) J. Catal. 109: 463, Siew Hew Sam et al. (1990) J. Catal.123:417, and Stern et al. (1997) Appl. Catal. A: General, 153:21.Various magnesium vanadates are reported to yield propylene withparticularly high selectivity and, when a second oxidic phase (Sb₂O₄) isadded, selectivities of up to 95% have been achieved. Carrazan et al.(1997) ACS Symp. Ser. 638:223. The mechanisms by which the oxidativedehydrogenation occurs and the interactions responsible for highselectivity are yet to be identified.

Other than vanadium-based catalysts, several molybdenum and niobiumcatalysts have also been investigated for use in oxidativedehydrogenation. See, for example, Breitescheidel et al. (1991) Chem.Mater. 3:559, Geenen et al. (1982) J. Catal. 77:499 and Toreis et al.(1987) J. Catal. 108:161. Bettahar et al. (1996) Appl. Catal. A: General145:1 and Kim et al. (1991) Appl. Catal. 70:175 discloses thatmolybdates of, for example, nickel or cobalt, yield acrolein insubstantial amounts, thereby decreasing the selectivity towardpropylene. Niobium oxide, particularly in combination with vanadium ormolybdenum, has been shown to have high selectivity for propylene in theoxidative dehydrogenation of propane in Savary et al. (1997) J. Catal.169:287.

Catalyst compositions without molybdenum or vanadium have also beenknown to perform oxidative dehydrogenation. Ji et al. (1996) Catal.Lett. 39:247 and Wang et al. (1995) J. Catal. 151:155 discuss theselectivity of combinations of lanthanum oxide, alkaline earth metal,and alkali metal. Unfortunately, temperatures in excess of 400° C. arerequired for any of the above-mentioned catalysts to have significantactivity and such temperatures result in decomposition of the propyleneoxide.

Catalysts composed of lanthanum carbonate and chromium oxide have beenshown to be active and selective at lower temperatures but have onlybeen used in the oxidative dehydrogenation of isobutane, see Hoang etal. (1997) J. Catal. 171:313. Carbonate-supported catalysts arecurrently used in ethylene epoxidation and often contain reduced silverand an alumina carrier. Catalysts of this nature have been described inU.S. Pat. No. 4,248,740 to Mitsuhata et al. and U.S. Pat. No. 4,342,667to Armstrong et al.

It has now been unexpectedly discovered that a highly selective catalystcapable of “one-pot” conversion of an alkane to an alkylene oxide can beobtained by using an alkaline earth metal carbonate as a support incombination with a rare earth metal promoter. Also surprising is thefinding that such catalysts are capable of the selective oxidativedehydrogenation of alkanes at temperatures under 400° C.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the invention to provide aprocess for the conversion of an alkane to an alkylene oxide attemperatures less than 400° C.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing, or may be learned by practice of the invention.

In a first embodiment, a novel process for the conversion of alkane toalkylene oxide is provided wherein an alkane and oxygen-containing gasfeedstream contacts an alkaline earth metal carbonate-supported silvercatalyst comprised of a catalytically effective amount of silver, and apromoting amount of a lanthanide metal promoter, an alkali metal halide,an alkali metal nitrate, and an optional transition metal promoter.

In another embodiment of the invention, a novel process for theconversion of propane to propylene oxide is provided wherein a propaneand oxygen-containing gas feedstream contacts an alkaline earth metalcarbonate-supported silver catalyst that has a catalytically effectiveamount of silver, and a promoting amount of a lanthanide metal promoter,an alkali metal halide, an alkali metal nitrate, and an optionaltransition metal promoter.

In a further embodiment of the invention, a novel catalyst compositionis provided comprising an alkaline earth metal carbonate support, acatalytically effective amount of silver, an effective promoting amountof a lanthanide metal promoter, an effective promoting amount of analkali metal halide, an effective promoting amount an alkali metalnitrate, and a promoting amount of a transition metal promoter.

In yet another embodiment of the invention, a novel process for theconversion of alkane to alkene is provided comprising contacting, at atemperature in the range of approximately 200° C. to 400° C., afeedstream, comprised of alkane and an oxygen-containing gas, and asupported silver catalyst, comprised of an inert refractory solidsupport comprised of alkaline earth metal carbonate, a catalyticallyeffective amount of silver, an effective promoting amount of a halideanion, an effective promoting amount of a rare earth metal promoter, aneffective promoting amount of a sodium promoter, and an optionaleffective promoting amount of a transition metal promoter.

In still another embodiment of the invention, a novel process for theconversion of propane to propylene is provided comprising contacting, ata temperature in the range of approximately 200° C. to 400° C., afeedstream, comprised of propane and an oxygen-containing gas, and asupported silver catalyst comprised of an inert refractory solid supportcomprised of alkaline earth metal carbonate, a catalytically effectiveamount of silver, an effective promoting amount of a halide anion, aneffective promoting amount of a rare earth metal promoter, an effectivepromoting amount of a sodium promoter, and an optional effectivepromoting amount of a transition metal promoter.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 presents a precipitation curve obtained during catalystsynthesis, as described in Example 17.

DETAILED DESCRIPTION OF THE INVENTION Definitions and Nomenclature

Before the present compounds, compositions, and methods are disclosedand described, it is to be understood that, unless otherwise indicated,this invention is not limited to specific support structures, reagents,methods of preparation, or the like, as such may vary. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only and is not intended to belimiting.

It must be noted that, as used in the specification and the appendedclaims, the singular forms “a,” “an,” and “the” include plural referentsunless the context clearly dictates otherwise. Thus, for example,reference to “a promoter” includes one or more promoters; reference to“a support” includes one or more supports, and the like.

The term “alkali metal” refers to elements of Group 1 of the PeriodicTable, i.e., lithium, sodium, potassium, rubidium, cesium, and francium.

The term “alkali earth metal” refers to elements of Group 2 of thePeriodic Table, i.e., beryllium, magnesium, calcium, strontium, barium,and radium.

The term “rare earth metal” refers to elements of the lanthanide andactinide series of the Periodic Table.

For the purposes of this invention, the term “conversion” is taken tomean the mole percent of propylene lost from the feed stream as a resultof reaction. Likewise, the phrase “selectivity to propylene oxide” istaken to mean the mole percent of reacted propylene that is used to formpropylene oxide. The conversion and selectivity of the process of thisinvention can vary over a wide range. Process variables influencingconversion and selectivity include temperature, flow rate, concentrationof oxygen, and concentration of propylene. Generally, as theconcentration of propylene in the feed stream decreases, the conversionof propylene and the selectivity for propylene oxide decrease as well.

A “support” is a carrier that comprises the catalytically activecomponents of a supported, i.e., heterogeneous, catalyst. In the presentcatalyst, the support is comprised of an alkali earth metal carbonate.

A “promoter” means a component that provides an improvement in one ormore of the catalytic properties of the catalyst, e.g., selectivity,activity, conversion, stability, and yield, as compared to a catalystnot containing the promoter. “Effective promoting amount” means anamount of a promoter sufficient to yield the above-mentionedimprovement.

“Halo” or “halogen” refers to fluoro, chloro, bromo, or iodo, andusually relates to halide substitution for a hydrogen atom in an organiccompound. Of the halides, chloro and fluoro are generally preferred.

“Optional” or “optionally” means that the subsequently described eventor circumstance may or may not occur, and that the description includesinstances where said event or circumstance occurs and instances where itdoes not. For example, the phrase “catalyst containing an optionaltransition metal anion” means that a transition metal anion may or maynot be present and that the description includes catalysts that comprisea transition metal anion and catalysts that do not.

The phrase “redox half-reaction” as used herein refers to half-reactionssuch as those found in equations presented in tables of standardreduction or oxidation potentials, also known as standard or singleelectrode potentials, of the type found in, for instance, the Handbookof Chemistry and Physics, CRC Press, 1995, pages D155-162.

As used herein, all references to the Periodic Table of the Elements andgroups thereof is to the version of the table published by the Handbookof Chemistry and Physics, CRC Press, 1995, which uses the IUPAC systemfor naming groups.

The Novel Process

The primary embodiment of the invention relates to a method ofconverting alkane into alkylene oxide using alkali earth metal carbonatesupported catalysts. Such catalysts are fully described below and inU.S. Pat. No. 6,392,066, entitled “Epoxidation of Olefins UsingLanthanide-Promoted Silver Catalysts”. The method comprises contactingan alkane with oxygen in the presence of the catalyst composition underconditions such that alkylene oxide is formed. The method of theinvention is of significant utility in the conversion of propane topropylene oxide but also is suitable for use with other alkanesincluding, but not limited to, ethane, butane, pentane, and the like.

While not wishing to be limited to a single theory, it is believed thatthe conversion of an alkane to alkylene oxide proceeds via initialoxidative chlorination of the alkane, followed by conversion of thechlorinated intermediates to alkene and subsequent epoxidation of thealkene to form an alkylene oxide. Evidence supporting the theory thatoxidative chlorination is the initial reaction pathway is found bysubstituting chlorinated species for alkane in the feed stream andobservation of alkene and alkylene oxide in the product stream. Inpropane-based applications, both 1- and 2-chloropropane are suchintermediates and, under certain circumstances, may be observed in theproduct stream.

The oxygen employed in the aforementioned process may be obtained fromany gas containing molecular oxygen, such as air, commercially pureoxygen, or other substance that, under the conditions necessary foroxidative dehydrogenation, both exists in a gaseous state and formsmolecular oxygen. The alkane and oxygen are present as an alkane andoxygen-containing gas feedstream in amounts sufficient to allowformation of the alkylene oxide. The concentration of alkane, such asethane or propane, in the feedstream is preferably in the range of about0.1% to about 25%, with concentrations ranging from about 1% to about20% being preferred and concentrations ranging from about 2% to about55% being most preferred. Similarly, the concentration of oxygen in thefeedstream is in the range of about 0.1% to about 25%, withconcentrations ranging from about 1% to about 20% being preferred andconcentrations ranging from about 2% to about 15% being most preferred.

The feedstream may also contain, but preferably does not contain, agaseous efficiency-enhancing member of a redox-half reaction and/or agas phase halogen compound, such as an alkyl halide. The gaseousefficiency-enhancing materials are compounds containing an elementcapable of existing in more than two valence states, preferablynitrogen, and another element that is preferably oxygen. Examples ofgaseous efficiency-enhancing members of a redox-half reaction pairinclude, but are not limited to, at least one of NO, NO₂, N₂O₄, N₂O₃, orany gaseous substance capable of forming one of the aforementionedgases, particularly NO and NO₂, under dehydrogenation conditions. NO ismost preferred. The gaseous efficiency-enhancing member of a redox-halfreaction is typically not present and is not necessary for conversion ofthe alkane into an alkylene oxide. If present in the feedstream,however, the gaseous efficiency-enhancing member is present inconcentrations ranging from about 0.1 ppm to about 2,000 ppm.Concentrations ranging from about 1 ppm to about 1000 ppm are preferredand concentrations ranging from about 50 ppm to about 500 ppm are mostpreferred.

Gas phase halogen compounds are preferably not included in thefeedstream. If included, however, such a gas phase halogen compound ispreferably an organic halide, saturated or unsaturated, such as1-chloropropane, 2-chloropropane, ethylene dichloride, ethyl chloride,vinyl chloride, methyl chloride and methylene chloride. Ethyl chloride,1-chloropropane, and 2-chloropropane are preferred. If used, the gasphase halogen compound is generally present in concentrations rangingfrom about 0.1 ppm to about 2,000 ppm. Concentrations ranging from about1 ppm to about 1000 ppm are preferred and concentrations ranging fromabout 50 ppm to about 100 ppm are most suitable.

The remainder of the feedstream may be made up of an inert gas such asargon, nitrogen or helium as a ballast or diluent. Varying amounts ofcarbon dioxide and water vapor may also be present, depending uponwhether means have been provided to remove such substances from thefeedstream components. It is preferred that no carbon dioxide be presentin the feedstream as the inclusion of carbon dioxide results in lowerselectivity.

The reactants can be contacted with the catalyst in any suitablereactor. Preferred are tubular stainless steel reactors designed towithstand the pressure and temperature of the reaction. While thereaction can take place in either the gaseous phase or in a liquidsolvent, contact in the gaseous phase is preferred. The oxygen/alkanefeed stream is preferably preheated to a temperature approximately thatof the reaction temperature. The reaction can be conducted at anyoperable temperature upon contacting the catalyst and the reactants.Generally, suitable reaction temperatures are above 50° C. and preferredtemperatures are in the range of about 200° C. to about 400° C., withtemperatures in the range of about 250° C. to about 350° C. mostpreferred. The pressure and temperature should be adjusted to achieveoptimal results for the particular catalyst and feedstream being used.Generally, pressures range from about 1 atmosphere to about 30atmospheres with atmospheric pressure being preferred.

The duration of time the reactants must remain in contact with thecatalyst must be sufficient to allow for oxidative chlorination of thealkane, halodehydrogenation of the chlorinated product, and epoxidationof the alkene intermediate produced by the halodehydrogenation.Generally, the duration of the contact varies according to the size ofthe reactor and the amount of catalyst used. Contact time is controlledby variation of the gas hourly space velocity of the feedstream as itpasses through the reactor. Typically, space velocities in the range offrom about 10 hr⁻¹ to about 15,000 hr⁻¹ are suitable. Space velocitiesin the range of from about 100 hr⁻¹ to about 6,000 hr⁻¹ are preferredand space velocities in the range of from about 500 hr⁻¹ to about 3,000hr⁻¹ are most preferred.

If propane is used in the feedstream, the propylene oxide reactionproduct is a useful industrial intermediate, particularly in themanufacture of urethane polymers. Propylene oxide is also useful in theproduction of propylene glycol, which is used to form plastics, and inthe production of propene glycol ethers, which are used as solvents. Thealkene reaction product from other alkane feedstocks can be used togenerate other alkylene oxides, and any haloalkyl by-products can beused to regenerate the catalyst, as will be appreciated by those skilledin the art.

Typically, the catalyst of this invention produces an alkane conversionof at least about 1 percent. Preferably, the catalyst produces aconversion greater than about 5 percent, more preferably a conversiongreater than about 8 percent, and most preferably, greater than about 10percent. Also, the catalyst of this invention produces a selectivity toalkylene oxide greater than about 1 percent, preferably greater thanabout 2 percent, and more preferably greater than about 5 percent.Selectivity for alkenes ranges from about 10% to about 40%.

The Catalyst

The catalysts of the invention are alkaline earth metal carbonatesupported, silver catalysts that incorporate a promoting amount of arare earth metal promoter, a halide promoter, an alkali metal, and analkali metal nitrate. The alkaline earth metal carbonate may be anycarbonate of any element of Group 2. Suitable carbonates are described,for example, in Canadian Patent No. 1,282,772 to Thorsteinson andinclude, but are not limited to, calcium carbonate, barium carbonate,strontium carbonate and magnesium carbonate. Calcium and bariumcarbonates are preferred. The alkaline earth metal carbonate mayconstitute from about 40% w/w to about 60% w/w of the catalystcomposition. Preferably, the alkaline earth metal carbonate is presentin the range of about 45% w/w to about 55% w/w of the catalystcomposition.

The silver is generally, although not necessarily, in the form of silvercarbonate and is present in the range of about 20% w/w to about 50% w/w.Preferably, the silver is present in the range of about 25% w/w to about45% w/w of the catalyst.

The rare earth metal promoter can be selected from any of the elementsof the lanthanide series, i.e., atomic number 57 to atomic number 70.Suitable promoters include, but are not limited to, lanthanum, cerium,praseodynium, gadolinium and erbium. Lanthanum is preferred. The rareearth metal promoter is present in amounts in the range of about 0.1%w/w to about 20% w/w of the catalyst composition. The rare earth metalpromoter is preferably present in amounts in the range of 1% w/w toabout 15% w/w and most preferably present in amounts in the range ofabout 5% w/w to about 10% w/w of the catalyst.

The catalyst is also infused with an alkali metal nitrate in addition tothe alkali metal nitrates formed by the recombination of the nitrate andalkali salts used during the synthesis of the catalyst, as will bediscussed below. The metal anion in the alkali metal nitrate may beselected from any of the elements of Group 1. Preferred alkali metalanions include sodium and potassium, with sodium most preferred. Thealkali metal nitrate is generally present in an amount sufficient toachieve an amount of alkali in the final catalyst in the range ofbetween about 0.1% w/w to about 2% w/w, preferably between about 0.3%w/w and 0.7% w/w, and most preferably about 0.5% w/w.

Alkali metal halide promoters are also present in the catalyst. Suchpromoters may be added to the catalyst composition in the form an alkalimetal halide or other soluble halide compound, i.e., HCl. Suitablealkali metal halide promoters include, for example, sodium chloride,sodium bromide, potassium chloride and potassium bromide. Preferredalkali metal halides are sodium chloride and sodium bromide, with sodiumchloride most preferred. The halide promoter may be present in an amountranging from about 0.005 to about 0.05 g Cl/g Ag, preferably from about0.01 g Cl/g Ag to about 0.02 g Cl/g Ag. In terms of molar ratios, thisrepresents an Cl/Ag molar ratio of about 0.015 mol Cl/mol Ag to about0.15 mol Cl/mol Ag, preferably between about 0.03 mol Cl/mol Ag to about0.06 mole Cl/mole Ag.

Optionally, the catalyst may be infused with a transition metal anion inthe form of a carbonate or nitrate in addition to the alkali metalcarbonates and nitrates formed by the recombination of the carbonate,nitrate and alkali salts used during the synthesis of the catalyst, aswill be discussed below. The transition metal anion may be selected fromany of the elements of Groups 3, 4, 5, 6, 7, and 8. Preferred transitionmetal anions include chromium, magnesium, and copper. When included, thetransition metal anion is present in an amount sufficient to achieve anamount of transition metal in the catalyst in the range of between about0.1% w/w to about 15% w/w, preferably between about 1% w/w and 10% w/w.

The catalyst composition may also contain an additional support element.Suitable supports include, but are not limited to, alumina, silica,titania, alkaline earth metal oxides, rare earth oxides, and mixtures ofthe above. Preferred catalysts do not contain additional supportelements.

The catalyst compositions may be formed using standard precipitationmethods. Such precipitation methods are well known in the art, see, forexample, U.S. Pat. No. 5,625,084 to Pitachai et al., U.S. Pat. No.3,3836,481 to Kajimoto et al., and U.S. Pat. No. 5,618,954 to Boeck etal. A basic solution containing an alkali metal carbonate and an alkalimetal hydroxide is reacted with an aqueous precursor solution containinga suitable alkaline earth metal salt, a silver salt, a rare earth metalpromoter, an alkali metal nitrate and an alkali metal halide promoterand, optionally, a transition metal carbonate as discussed above. Theprecursor solution may be acidified using nitric acid. Acidification ofthe precursor solution assists in the dissolution of the precursor saltsand the solution may be acidified to a pH of about 3. Such acidificationgenerally takes place prior to the addition of the basic solution. Itshould be noted that although other acids, such as HCl, may be used toacidify the solution, sulfuric acid or phosphoric acid are not favoredas these acids may result in the formation of sulfates and phosphateswhich are not desired in the final catalyst. Organic acids are alsodisfavored. If HCl is used to acidify the precursor solution, it mayonly be added in limited amounts as the total concentration of halide inthe catalyst should be in an amount ranging from about 0.005 g Cl/g Agto about 0.05 g Cl/g Ag, preferably from about 0.01 g Cl/g Ag to about0.02 g Cl/g Ag, as discussed above.

In one precipitation method, method A, the basic solution is injected bypump into the precursor solution to form a precipitation solution. ThepH of the precipitation solution is monitored and the injection of thebasic solution is terminated when the pH of the precipitation solutionindicates that all silver and alkaline earth metals have precipitatedout, generally at a pH of about 10. In a second precipitation method,method B, the basic solution and the precursor solution aresimultaneously added to a separate water-containing precipitation vesselforming the precipitation solution therein. The pH of the precipitationsolution is monitored and the injection rate of the basic solution iscontrolled to maintain the pH of the precipitation solution at a desiredlevel, generally in the range of about pH 10 to about pH 12. The rate ofprecursor injection is held constant, in the range of about 10 mL/h toabout 1000 mL/h.

During precipitation, the carbonate ion from the alkali metal carbonateinteracts with the alkaline earth metal ion and the silver ion containedin the precursor solution, forming the silver-containing alkaline earthmetal carbonate support, which then precipitates out of solution.Similarly, the alkali metal ion from the basic solution and the nitrateand carbonate ions from the precursor and basic solutions interact toform alkali metal nitrates and carbonates. Formation of alkalicarbonates from these substituents will be most favored. Promotingamounts of the rare earth metal promoter and alkali metal nitrate andalkali metal halide promoters, if included, are also contained in theresulting precipitate.

When method A is used, the components of the catalyst precipitatesequentially. It is observed that the rare earth metal promoter and thesilver co-precipitate out of the solution first, as hydroxides,hydroxycarbonates, and/or carbonates, at a pH of about 7. This isfollowed by the alkaline earth metal carbonate, which precipitates fromabout pH 7.5 to about pH 8.5. When method B is used, the variouscomponents co-precipitate out of solution.

The resulting precipitate is then filtered, dried and calcined atsufficient temperature and for a sufficient time to reduce the silverprecursor without decomposing the alkali nitrates or alkali halogenpromoters, generally at about 300° C. to about 350° C. for 10 to 20minutes or less. It is important to note that the precipitate is notwashed before drying and calcination in order to maintain the level ofalkali present. The activity of the catalyst may be tested using aquartz flow reactor attached to an automatic sampling valve for GCanalysis.

The alkali metal carbonate used in the basic solution may be anyselected from potassium carbonate, sodium carbonate, rubidium carbonate,or cesium carbonate, or mixtures thereof. Potassium carbonate and sodiumcarbonate are preferred and sodium carbonate is most preferred.

The alkali metal hydroxide is used to control the pH of theprecipitation solution and to provide additional alkali ions. Suitablealkali hydroxides are sodium hydroxide and potassium hydroxide. Sodiumhydroxide is preferred.

The alkaline earth metal salt may be any salt that will not adverselyreact with the other components utilized. Suitable salts include, butare not limited to, nitrates, nitrites, propionates, sulfates,chlorates, perchlorates and chlorites. Examples of specific alkalineearth metal salts include, but are not limited to, calcium nitrate,barium nitrate, magnesium nitrate, strontium nitrate, calcium sulfate,barium sulfate, magnesium sulfate, strontium sulfate. Barium nitrate andcalcium nitrate are preferred and barium nitrate is most preferred.

The silver salt may be any salt that will not adversely react with theother components utilized. Suitable salts include, but are not limitedto, nitrates, nitrites, propionates, sulfates, chlorates, perchloratesand chlorites. Examples of specific silver salts include, but are notlimited to, silver nitrate, silver sulfate, silver chlorate, silversulfate, silver nitrite, silver propionate, silver perchlorate, silverchlorite and mixtures thereof. Silver nitrate is preferred.

The transition metal anion used in the precursor solution may beselected from chromium hydroxy carbonate, Cu(NO₃)₂, Mg(NO₃)₂, ormixtures thereof. Chromium hydroxy carbonate and Mg(NO₃)₂ are mostpreferred

Experimental

It is to be understood that, while the invention has been described inconjunction with the preferred specific embodiments thereof, theforegoing description is intended to illustrate and not limit the scopeof the invention. Other aspects, advantages and modifications within thescope of the invention will be apparent to those skilled in the art towhich the invention pertains.

All patents, patent applications, and publications mentioned herein arehereby incorporated by reference in their entireties.

EXAMPLE 1 Preparation of Catalyst

A catalyst of the invention was alternatively prepared according to thefollowing procedure. A basic solution containing 1 g NaOH and 15 g ofK₂CO₃ in 200 mL of purified water was prepared. A precursor solutionhaving a pH of 4.0 was prepared by first dissolving 3.5 g Ba(OH)₂ inHNO₃ and then dissolving 1 g La(NO₃)₂.6H₂O, 30 mg NaCl, 5 g NaNO₃, and2.5 g AgNO₃. The basic solution was then injected into the precursorsolution using a Gilson peristaltic pump at a rate of 50 mL/h withvigorous stirring. A brownish precipitate formed. The precipitation wascomplete in 50 minutes and injection of the basic solution was stoppedat a final pH of 10. The precipitated catalyst was filtered and dried at160° C. for 30 minutes. The catalyst was then calcined in flowing air at325° C. for 15 minutes.

EXAMPLE 2 Alternative Preparation of Catalyst

A catalyst of the invention was alternatively prepared according to themethod of Example 1. A basic solution containing 1 g NaOH and 11.5 g ofNa₂CO₃ in 200 mL of purified water was prepared. A precursor solutionhaving a pH of 4.0 was prepared by first dissolving 3.5 g Ba(OH)₂ inHNO₃ and then dissolving 1 g La(NO₃)₂.5H₂O, 1.5 g Cr(NO₃)₂. 9 H₂O, 50mg, NaCl, 5 g NaNO₃, and 2.5 g AgNO₃. The basic solution was theninjected into the precursor solution using a Gilson peristaltic pump ata rate of 50 mL/h with vigorous stirring. A dark blue precipitateformed. The precipitation was complete in 50 minutes and injection ofthe basic solution was stopped at a final pH of 10. The precipitatedcatalyst was filtered and dried at 160° C. for 30 minutes. The catalystwas then calcined in flowing air at 300° C. for 30 minutes. A brightyellow catalyst resulted.

EXAMPLE 3 Alternative Catalyst Preparation

A catalyst of the invention was alternatively prepared according to themethod of Example 1. A basic solution containing 1 g NaOH and 11.5 g ofNa₂CO₃ in 200 mL of purified water was prepared. A precursor solutionhaving a pH of 4.0 was prepared by first dissolving 3.5 g Ba(OH)₂ inHNO₃ and then dissolving 1 g La(NO₃)₂.5H₂O, 1.5 g Mg(NO₃)₂.H₂O, 50 mgNaCl, and 2.5 g AgNO₃. The basic solution was then injected into theprecursor solution using a Gilson peristaltic pump at a rate of 50 mL/hwith vigorous stirring. The precipitation was complete in 50 minutes andinjection of the basic solution was stopped at a final pH of 10. Theprecipitated catalyst was filtered and dried at 160° C. for 30 minutes.The catalyst was then calcined in flowing air at 300° C. for 30 minutes.

EXAMPLE 4 Activity Testing

The activity of all of the catalysts of Examples 1-3 was testedaccording to the following method. 2.5 g of 20 to 48 mesh catalyst wasplaced in a quartz flow reactor, with a flow of 40 mL/min of 5% propane,5% O₂, and balance He, at atmospheric pressure. Space velocity was 1200h⁻¹. The hydrocarbon/oxygen gas mixture composition was controlled usingelectronic mass flow controllers. The effluent from the reactor was ledthrough a heated transfer line into an automating sampling valve for GCanalysis. The GC includes column switching between a 1-foot HP13×molecular sieve column and an Alltech Hayesep D 100/12 column, tooptimize the separation of both fixed gases and higher hydrocarbons. Aflame ionization detector (FID) was place in series with a thermalconductivity detector (TCD) to provide better quantification ofhydrocarbons, especially those present in low concentrations (below 1vol. %). The response factors for the various gases (CO, CO₂, C₃H₈,C₃H₆, propylene oxide, and C₂ species) for both detectors weredetermined, for quantification purposes and to allow calculation of massbalances. The composition of product streams was also confirmed by massspectroscopy, using a Dycor Quadlink spectrometer.

EXAMPLES 5-9 Catalyst Conversion and Selectivity

The following Ag/Cl/NaNO₃/La/BaCO₃ catalysts were prepared according tothe methods described in Examples 1-3 and their activity assessedaccording to the method described in Example 4. The results presented inTable 2 were obtained after 2 hours of reaction time at 280° C.

TABLE 1 Trans- Pro- ition pane Selectivity Metal Conver- Propy-Propylene Example Promoter sion lene Oxide 1-Chloropropane 5 11% 16% 5%6% 6  5% Cr* 11% 23% 5% 7% 7 10% Cr*  8% 28% 6% 9% 8 Cu  5% 29% 2% 7% 9Mg 5° C. 30% 4% 7% *Cr-content presented as percentage chromiumhydroxy-carbonate

EXAMPLE 10 Activity Testing Using Increased Feed Ratio

The activities of all of the catalysts of Examples 1-3 were tested asdescribed in Example 4, but a feedstream of 11% propane, 18% O₂, andbalance He, at atmospheric pressure was used instead.

EXAMPLES 11-13 Catalyst Conversion and Selectivity

The following catalysts were prepared according to the methods describedin Example 2 with 10% Cr as a percentage chromium hydroxy-carbonate andusing Ba, Ca, and Sr supports. The activity of the catalysts wasassessed according to the method described in Example 10. The resultspresented in Table 2 were obtained after 20 and 300 minutes of reactiontime at 280° C.

TABLE 2 CATALYST ACTIVITY USING INCREASED FEED RATIO Ex- Selectivity am-Catalyst Time Propane Pro- Propylene 1- ple Support (min) Conversionpene Oxide Chloropropane 11 BaCO₃  20  10% 23%   8%   7% 12 BaCO₃ 300 21%  6% 0.8% 0.9% 13 CaCO₃  20 7.4% 26%   4%   3% 14 CaCO₃ 300 2.8% 23%  4%   5% 15 SrCO₃  20 5.6% 39%   4%  15% 16 SrCO₃ 300 1.3% 37%   4% 16%

EXAMPLE 17 Precipitation Curve

The following precipitation curve (FIG. 1) was obtained using the methodof Examples 1-3. The precipitation was conducted by injecting a solutioncontaining 4 g/L NaOH and 60 g/L K₂CO₃ at 50 mL/h into a 250 mL solutionof 5 g Ca(NO₃)₂.4H₂O and 2.5 g AgNO₃, with vigorous stirring. The pHchanges as a function of the amount of base solution injected and theamount of remaining Ag⁺ and Ca⁺ present in the solution. When theprecipitation rate equals the rate of base(carbonate) injection, the pHattains a constant value.

We claim:
 1. A process for the conversion of alkane to alkene comprising, contacting at a temperature in the range of approximately 200° C. to 400° C.: (a) a feedstream comprising alkane and an oxygen-containing gas; and (b) a supported silver catalyst comprising (i) an inert refractory solid support comprised of alkaline earth metal carbonate, (ii) a catalytically effective amount of silver, (iii) an effective promoting amount of a halide anion, (iv) an effective promoting amount of a rare earth metal promoter, (v) an effective promoting amount of a sodium promoter, and (vi) an optional effective promoting amount of a transition metal promoter.
 2. A process of claim 1, wherein the catalyst is prepared by precipitation.
 3. A process of claim 1, wherein the feedstream is essentially free of carbon dioxide.
 4. The process of claim 1, wherein the rare earth metal promoter is lanthanum.
 5. The process of claim 1, wherein the alkaline earth metal carbonate support is selected from the group consisting of strontium carbonate, calcium carbonate, barium carbonate and mixtures thereof.
 6. The process of claim 5, wherein the alkaline earth metal carbonate is a barium carbonate support.
 7. A process for the conversion of propane to propylene comprising contacting at a temperature in the range of approximately 250° C. to 350° C.: (a) a feedstream comprising propane and an oxygen-containing gas; and (b) a supported silver catalyst comprising (i) an inert refractory solid support comprised of alkaline earth metal carbonate, (ii) a catalytically effective amount of silver, (iii) an effective promoting amount of a halide anion, (iv) an effective promoting amount of a rare earth metal promoter, (v) an effective promoting amount of a sodium promoter, and (vi) an optional effective promoting amount of a transition metal promoter.
 8. A process of claim 7, wherein the catalyst is prepared by precipitation.
 9. A process of claim 7, wherein the feedstream is essentially free of carbon dioxide.
 10. The process of claim 7, wherein the rare earth metal promoter is lanthanum.
 11. The process of claim 7, wherein the alkaline earth metal carbonate support is selected from the group consisting of strontium carbonate, calcium carbonate, barium carbonate and mixtures thereof.
 12. The process of claim 11, wherein the alkaline earth metal carbonate support is a barium carbonate support.
 13. The process of claim 7, wherein the alkali metal halide is sodium chloride.
 14. The process of claim 7, wherein the sodium promoter is sodium nitrate.
 15. The process of claim 7, wherein the promoting amount of transition metal promoter is present.
 16. The process of claim 15, wherein the transition metal promoter is selected from the group consisting of chromium, copper and magnesium.
 17. The process of claim 16, wherein the transition metal promoter is chromium. 